1. Field of the Invention
The present invention relates to a selective-calling radio receiver such as a pager and more particularly, to a selective-calling radio receiver equipped with a vibrator for vibration warning to a user.
2. Description of the Prior Art
Conventional selective-calling radio receivers of this sort were disclosed in the Japanese Non-Examined Patent Publication Nos. 4-281630 published in October 1992 and 5-191334 published in July 1993. In these conventional receivers, a dc power generated by a dc power supply (for example, a dry battery) is intermittently supplied to a vibrator under the operation of a switching transistor, thereby generating an intermittent vibration of the vibrator. The supplied power to the vibrator has a substantially square waveform and is caused by the switching operation of the transistor. The vibrator has a pulse motor and a vibration plate eccentrically fixed to the rotating shaft of the motor.
With the conventional selective-calling radio receivers described above, since a comparatively large current is necessary for the dc power supply to drive the vibrator, a dry battery, which can provide a large supply current, is often used as the power supply. However, the electromotive force of the dry battery tends to decrease with the discharge time and as a result, the following problem will occur:
Specifically, because of the electromotive force decrease of the dry battery, the driving power for the warning vibrator tends to decrease and accordingly, the vibration strength of the vibrator also decreases with the discharge time of the dry battery. For example, when the amplitude of the square-wave driving voltage supplied from the dry battery decreases from 1.5 V to 1.1 V due to the driving power lowering of the dry battery, the vibration strength of the vibrator may tend to decrease by 46% of the normal vibration strength. Such the decrease of the vibration strength will increase the danger that the receiver user does not notice the vibration warning.